Tommy Lynn Sells smiled when he said killing was the best feeling he'd ever had in his life. Better than anything. Like a shot of heroin straight to the brain - and he spent decades chasing that high. He estimated he killed somewhere between 20 and 50 people. He didn't know the exact number. There were too many to count. Texas executed him in 2014. He killed a pregnant woman 7.5 months along. She went into labor during the attack. He killed the newborn too. Her husband's body turned up the next day - shot three times in the head. In the video below, ABC News sits down with Tommy Lynn Sells on death row - and what comes back across the glass is something the interviewer says she was grateful the glass was there for. He said he didn't know what love was. Said sorry was a word he didn't use. Said when he looked at the camera he saw hate and that's all he'd ever known. The interviewer asked what would happen if she said something he didn't like. He said her head would go down into the concrete and crack like a coconut. Then he smiled again. What finally caught him was a 10-year-old girl who survived with a sliced throat. In 1999 he broke into a trailer, cut the throat of 13-year-old Katie Harris while her friend Crystal watched from the top bunk, then came back for Crystal too. Left her for dead. She described him to police from a hospital bed. It ended 15 years of killing. When asked if he had anything to say to the little girl who lived, he paused. Said she probably didn't want to hear what he had to say. Was Tommy Lynn Sells born without the part of the brain that makes killing feel wrong, or did something destroy it early enough that nothing could have stopped what came next?